1. Field of the Invention
This invention relates to a speed control method for printing press and a printing press practicing the method.
2. Description of the Prior Art
Generally, in a printing press, for example, a rotary mimeograph printing press, the printing speed, that is the rotation speed of a drum or plate cylinder can be set desirably by the operator when printing. Printing is started when the rotation speed of the drum reaches a set value or during a period of time in which the rotation speed of the drum changes increasingly before it reaches the set value.
However, in case where printing is started with the rotation speed of the drum reaching the set value, the time required for overall printing is disadvantageously prolonged, and where printing is started while the rotation speed of the drum is changing, there arises a problem that a sheet of printing paper is printed at different printing speeds because the rotation speed changes continuously, in particular, linearly throughout a period covering the initial rotation and the attainment of the set value, thus causing non-uniform density of picture in which a picture is more shaded at a portion printed at a low printing speed than at a portion printed at a high printing speed.
Another method has been available wherein the drum or plate cylinder is initially rotated at a speed set by the operator. In this case, when the set speed is high, a drive system is abruptly loaded and through repetitious operations, the drive system is degraded to lose durability and damaged in an extremity.
It is also desired in printing that when some abnormality (for example, mechanical lock) occurs in a load system on the printing press and the load increases abruptly, a drive motor be stopped quickly. Conventionally, an emergency stop device for the drive motor has been available which uses a fuse or a breaker that makes use of an increase in current attendant on an increased load to disconnect an electrical circuit. However, the fuse or breaker is dedicated to protecting the drive motor from overheating and consequent burning-out and is operated, in principle, with heat generated by an overcurrent. Accordingly, the emergency stop device delays in responding and has irregularity in operation time, failing to meet the requirement of emergency stop.
With the conventional printing press, therefore, as the load increases, generated torque is increased correspondingly, with the result that the motor is forced to increase torque thereby imposing excessive torque on a drive transmission system and in extremities there occur troubles that a belt for drive transmission is tensed so vigorously as to be flown off and gears are torn off. Since such breaking of the belt and gears usually proceeds within one second at the least and matures in advance of the activation of the low-response fuse and breaker, it is impossible to avoid such breaking.